Traditional telecommunications systems have evolved rapidly in recent years to encompass considerably more capability than simply delivering a telephony service. Internet technology based on data packets and associated transmission protocols, for example the Internet Protocol (IP), has now superseded the old space switching techniques and dedicated transmission resources during a telephone call or data transmission session.
The convergence of flexible delivery technologies has enabled services to be mixed in the same physical resource so that for example, in telephony networks, subscribers are now able to receive audio-visual content, including relayed broadcast television and radio services, as well as access internet resources, all via their telephone lines. Similarly, the expansion of wireless networks have enabled a user to receive a range of communications services over a wireless delivery channel.
A Digital Subscriber Loop (xDSL) modem/router, which is widespread in homes, acts as a common assembly point for all domestic communications traffic, such as telephony, computer-based web browsing, email, home shopping or streaming of video material. As will be appreciated by a skilled person, the capacity of the digital subscriber loop has to be shared among the services being provided to the subscriber at any particular time.
There is considerable interest in receiving “streamed” services which deliver a stream of video and/or audio material to users. The content can be live content or it can be recorded content which is requested on demand. In an Internet Protocol Television (IPTV) system, video and audio content is delivered to users via Internet Protocol (IP) delivery mechanisms. These types of services require a continuous and uninterrupted flow of data. A premises may have multiple terminals which are capable of receiving media content. The demands placed on the delivery channel can vary according to how many terminals require content at a particular time.
Streamed media content services can be provided by one of two methods: Unicast or Multicast. Unicast provides the subscribing receiver with a direct and unique two-way path through the delivery network all the way back to the media server supplying the required data stream. The main streaming activity is managed on a one-to-one basis between the receiver and the source server. The network between the source server and receiver typically comprises a series of intermediate servers installed at network nodes which are not directly involved in the service but only support the transfer of a packet stream. Typically the protocols used to support the transmissions are simple forms of Internet Protocol (IP) itself augmented by one or more higher layer protocols to provide flow control. These protocols extend across the span of the link between the source server and a given receiver. This method of distribution is wasteful of network capacity because, at busy times and for popular media streams, many copies exist in the network simultaneously and this contributes to network congestion.
A Unicast system can support Adaptive Bit Rate Streaming (ABR). This allows some form of rate adaptation. A given service is encoded at a selection of different bit rates (known as representations), with synchronised boundary points at defined locations (e.g. every 50 frames). For each representation, content between successive boundary points is converted into a discrete file. Clients fetch a segment of one of the representations in turn. If a higher or a lower bit rate is required, the next segment is fetched from one of the other representations. The segments are constructed such that there is no discontinuity in decoded pictures/audio if the client switches between representations at the boundary points. This system requires the unicast two-way path between source and receiver to request files and deliver the requested files.
Multicast makes more efficient use of bandwidth by sharing content streams among several receivers. Intermediate network routers are now more closely involved in the service delivery such that some control and management functions are delegated from the source server. This control is supported by more extensive protocols devised for this type of application such as Protocol Independent Multicast (PIM) and Internet Group Multicast Protocol (IGMP). When a new receiver requests a given media item, the network router system finds an existing stream of that content already in the network and directs a copy of it to that new receiver from an appropriately near network node. A new receiver has to be able to join this existing stream under controlled conditions that do not adversely affect existing receivers. Any receiver in this group also has to be able leave the stream, or pause its consumption, without affecting the others. There is now a clear separation of control and management from the content. This complicates the control function because several receivers will have different instantaneous bit rate and control needs and so the segmentation of the stream as it passes through a given network has to deal with the conflicting demands. These are managed directly between the receiving device itself and the nearest upstream router/server that will mediate control functions including any need to refer back to earlier network servers or even the original source server.
Currently, each TV service has to be set up with a conservative configuration. This can be a capped Variable Bit Rate (VBR) with a safe cap value. This is because it is not possible to change bit rate of a service on the fly without interrupting the TV service.
In summary, while unicast systems can support an adaptive bit rate of a streamed service over time, this comes at a significant cost in terms of resources of the delivery network to support a large number of unicast paths, and may require additional servers located near to clients of the network to maintain acceptable response times. Multicast systems can more efficiently use bandwidth resources of the network, but do not support adaptive bit rate needs of client devices.
The present invention seeks to provide an alternative way of distributing media content services, such as video services.